Interpretive Summary: Development of effective contol of Porcine Reproductive and Respiratory Syndrome (PRRS) has been slowed by the weak immune response to PRRS virus infection. When present in a herd PRRS is manifested by increased rates of abortions, stillbirths, mummifications, and chronic respiratory problems. Economically, PRRS costs the US pork industry $560 million annually. The weak protective response leads to persistent viral infection in a subset of pigs. The work reported here was designed to invetigate the genetic control and mechanisms regulating the intensity and timing of the early cytokine responses to PRRSV infection and their role in persistence. It was hoped that this would give insight into the utility of cytokine responses as predictors of both final PRRSV burden and viral persistence. This work used samples collected as part of the “Big Pig” project in which 109 Large White/Landrace cross bred pigs were infected with PRRSV and followed for 202 days. We tested gene and protein expression in immune tissues collected from pigs at day of kill and serum samples collected biweekly for up to 202 days post-PRRSV infection (dpi). To assess persistence we compared pigs that apparently cleared the viral infection from serum and tissues, the non-persistent (NP) pigs, to persistent (P) pigs which had virus in serum and/or tissues at kill. Gene expression studies in tracheobronchial LN (TBLN) samples collected at kill demonstrated that there is up regulation of cytokine expression, of interferon-gamma (IFNG) and associated T helper 1 (Th1) markers from 14 to 84 dpi, and T-regulatory, IL10, and but importantly no upregulation of innate markers (IFNA, IL1b, IL8) that are required for effective early immunity. At later time points (>112 dpi, the time chosen for persistence analyses) these immune genes were no longer differentially expressed and thus were uninformative for persistence studies. Moreover there were no differences in gene expression between NP and P tissues. We next asked whether serum immune cytokine (IL-1b, IL-8, IFN') levels early after infection might determine why certain pigs have persistent PRRSV infections. Statistical analyses indicated that innate cytokines IL-1b and IL-8 levels were up-regulated early after infection, and associated with a significant difference between NP and P pig levels at 14 dpi for IL-8; serum IFN-gamma levels peaked at 28 dpi. When analyzed together, variation in all 3 of the cytokines tested (IL-8, IL-1b and IFN-gamma) had a significant effect on virus level, accounting for ~84% of the variation observed, however, it is not known if this is due to a direct or indirect effect of these cytokines. These results indicate that while each cytokine individually has minor effects on persistency, the combination of cytokines could be used to potentially determine which pigs will have persistent virus infections. These analyses set the stage for current more detailed studies of genetic control of PRRS through the PRRS Host Gentics Consortium.

Technical Abstract:
Infection with Porcine Reproductive and Respiratory Syndrome virus (PRRSV) elicits a weak immune response that is weakly protective and results in persistent infection in a subset of pigs. We investigated the intensity and timing of the early cytokine responses to PRRSV infection to determine their utility as a predictor of final PRRSV burden and persistence. As part of the “Big Pig” project we tested gene and protein expression in immune tissues collected from pigs at kill and serum samples collected biweekly for up to 202 days post-PRRSV infection (dpi). We compared pigs that apparently cleared the viral infection from serum and tissues, the non-persistent (NP) pigs, to persistent (P) pigs which had virus in serum and/or tissues at kill. Gene expression studies in tracheobronchial LN (TBLN) samples collected at kill demonstrated that there is up regulation of expression of interferon-gamma (IFN') associated T helper 1 (Th1) markers from 14 to 84 dpi, of T-regulatory, IL10,and but no upregulation of innate markers (IFNA, IL1b, IL8). At later time points (>112 dpi, the time chosen for persistence analyses) these genes are no longer differentially expressed and thus are uninformative for persistence studies. We next asked whether immune cytokine (IL-1b, IL-8, IFN') levels early after infection determine why certain pigs have persistent PRRSV infections. Statistical analyses indicated that innate cytokines IL-1b and IL-8 levels were up-regulated early after infection, with a significant difference between NP and P pig levels at 14 dpi for IL-8; serum IFN' levels peaked at 28 dpi. When analyzed together, variation in all 3 of the cytokines tested (IL-8, IL-1b and IFN') had a significant effect on virus level, accounting for ~84% of the variation observed, however, it is not known if this is due to a direct or indirect effect of these cytokines. These results indicate that while each cytokine individually has minor effects on persistency, the combination of cytokines could be used to potentially determine which pigs will have persistent virus infections.